The E major seventh flat sixth chord is a 4-note chord consisting of the notes E, G#, C and D#.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as EM7b6.
Interactive piano diagram for the E major seventh flat sixth chord
Chords that are a subset of EM7b6. These contain less notes but all of them are included in EM7b6.